As offshore oil and gas production move to deeper waters where production costs are higher, it becomes necessary to develop smaller or secondary fields which do not justify the costs of design and installation of fixed production platforms. The number of "marginal" subsea oil fields grows rapidly with increasing water depth and makes the concept of developing a field with satellite subsea wells attractive.
Floating production systems employing ship-shaped vessels, barges or semi-submersible-type hulls have been used to obtain early production prior to construction of permanent, bottom-founded structures. Floating production systems have also been installed to produce "marginal" subsea reservoirs with one or two wells, reservoirs which would be too small to justify the costs of development with a bottom-founded structure.
One requirement for efficient exploitation of marginal fields is the possibility of wireline re-entry into a subsea well. Wireline servicing of a well is normally conducted from fixed platforms or heave compensated floating systems. In normal conditions, a relatively large deck space is required to place the wireline unit in appropriate proximity to the lubricator as is the case for on land use of wireline equipment.
If a field is to be developed with satellite subsea wells, a major difficulty is providing an economic way to re-enter the well. In the past, if downhole work was required to change out a gas lift valve, shift a sliding sleeve or the like, the only available options would be to incorporate "pumpdown" or "through-the-flow line" tool systems into the design of the subsea well or to mobilize a drilling rig to make a direct wireline re-entry into the satellite well from the water surface immediately above the well. Through-the-flowline systems are expensive and not very reliable, thus, they have not found great favor with oil field operators. Mobilizing a drilling rig for a wireline operation of short duration is obviously very expensive.
Adaptation of known designs for floating structures, such as semi-submersible hulls and spar buoys, to function as a single well service buoy do not provide adequate sea keeping characteristics for such application. Known semisubmersible designs provide wave pressure cancellation utilizing two vertically connected cylinders, the upper cylinder being of a smaller diameter so that the total force is minimal in the heave (vertical) direction at a specific wave frequency.
While the minimalization of heave response is helpful in maintaining a stable tension on a tensioned riser, such a design has little effect on the sway (to and fro) response of the buoy which is critical to minimizing angular deviation of a tensioned vertical riser. Common spar buoys similarly minimize heave response while surge and sway response is not adequately limited. Sway motions permitted by common catenary mooring of floating structures permit only general, imprecise location of buoys as navigational aids, unmanned weather stations and the like.